1. Field of the Invention
This invention generally relates to a resin composition for impregnating porous articles, and to a system for treating waste water from an impregnation process utilizing such resin composition.
2. Background and Description of the Related Art
Impregnation sealing of microporosity is a commonly used methodology in the art of forming a variety of articles, structural components, and assemblies, as for example castings, die castings, electronic components, powder metal parts, fiber-reinforced resin composites and other materials which exhibit porosity.
Originally, materials, manufacturing techniques, and casting designs were specified to minimize the occurrence of porosity in formed objects, based on the hypothesis that microporosity was structurally and functionally undesirable and its presence in formed articles embodied poor manufacturing. This approach severely limited design freedom, and resulted in significant rejection of parts exhibiting any substantial porosity characteristics.
This design strategy changed in the 1970's as a result of the energy crisis, which resulted in a major switch to lighter metals for structural applications. During this period many iron parts were changed to cast aluminum components, and many other parts were designed as die castings. This switch to ligther metals resulted in weight savings in many applications where energy consumption and power optimization were important, but created a new and persistent problem of microporosity in the light metal formed parts. The occurrence of microporosity is particularly acute in components formed from metal powder, and presents a significant obstacle to commercial utility, particularly when such porous parts are employed in fluid power systems and other liquid handling applications.
In order to overcome the deficiencies attendant the presence of microporosity in formed articles of the above-described types, impregnation sealing technology was developed, by which the porosity of the porous parts was impregnated with a sealant composition. Upon curing of the impregnated sealant, the resulting sealed part is suitable for use in fluid exposure applications, as well as facilitating plating, coating, and further processing of the formed article.
Among the impregnation sealing compositions which have been developed to date are self-curing anaerobic sealants and thermal curing sealants, as well as sealants which cure by both anaerobic and heat cure mechanisms.
Electronic encapsulating sealant/coating compositions, curable both anaerobically and with exposure to UV light, have also been developed for vacuum impregnation of electrical components such as transformers, wherein the encapsulating sealant is anaerobically cured inside the device and is cured on the outside surface with UV light to encapsulate the device. To effect a thorough outer surface curing of the sealant, such compositions typically contain a UV photoinitiator in concentrations substantially in excess of 5% by weight, based on the weight of the curable component thereof.
In addition, sealant/coating compositions have been developed for sealing of laminates, composite materials, and the like, containing macroscopic or gross voids into which the sealant/coating composition after surface application flows by capillary, or wicking, action. Generally, sealant/coating compositions employed in such applications are highly viscous in character, having a viscosity substantially greater than 1000 centipoise, as measured by the Cannon-Fenske viscosity determination method. One such conventional formulation, having a Cannon-Fenske viscosity of 4200 centipoise, contains 3.4 weight percent of a UV photoinitiator, based on the weight of curable component in the sealant/coating composition, to effect surface cure of the composition under UV radiation, in combination with internal anaerobic curing of the composition. The high viscosities of such compositions generally require long processing times for impregnation of microporosity.
As a consequence of external surface UV cure employed for the above-described electronic encapsulating sealant/coating compositions and capillary action impregnant compositions, no washing or other removal of excess surface sealant is required in these impregnation systems.
Among the previously developed impregnating compositions for sealing porous parts are the compositions disclosed in the patents identified and discussed below.
E. Neumann U.S. Pat. No. 3,672,942 discloses an anaerobic impregnant comprising a free-radical polymerizable acrylate ester monomer and free-radical polymerization initiator therefor, e.g., a hydroperoxide. The patent discloses utilizing an accelerator in the impregnant, such as aldehyde-amine condensation products, sulfur-containing free-radical accelerators, or organic compounds containing an oxidizable transition metal. This reference also discloses a vacuum impregnation process in which the porous article is placed in a vacuum vessel, followed by drawing of vacuum therein and covering the article with the disclosed anaerobic sealant so that upon release of vacuum, the sealant is forced into the evacuated porosity of the article. The surface of the impregnated article then is treated with the aforementioned polymerization accelerator to cure the sealant at the outer surface of the porous article.
U.S. Pat. No. 3,969,552 describes a washing proces for removing excess impregnant from the surface of the porous article after porosity impregnation. The disclosed impregnation composition comprises an acrylic anaerobic curing resin and a peroxy initiator therefor. The wash solution is an aqueous solution of a surfactant of specified formula. The patent further discloses that the aqueous surfactant solution may contain an accelerator to effect polymerization of the anaerobic sealant in the surface areas of the impregnated part being washed.
J. DeMarco U.S. Pat. No. Re. 32,240 describes a self-emulsifying anaerobic composition for porosity impregnation applications, comprising an anaerobically curing monomer such as an acrylic ester, a peroxy initiator therefor, e.g., a hydroperoxide or perester, an anionic or nonionic surfactant which is dissolved in the composition and renders it self-emulsifying upon mixing with water, and optionally an accelerator for the anaerobic polymerization, e.g., a sulfimide. The impregnant composition of this patent is described as being readily removed from surface areas of porous parts by simple aqueous rinse, even in difficult areas such as small blind holes in complex castings, from which it is difficult to remove excess anaerobic impregnant even with an agitiated water rinse.
M. L. Garcia, et al, U.S. Pat. No. 4,632,945 discloses an anaerobic sealant material comprising a (meth)acrylate monomer, a hydroperoxide or perester initiator, an acclerator having --SO.sub.2 NCO-- functionality, and a transition metal co-acclerator comprising a source of copper ion and an iron salt or ferrocenyl compound.
The above-described anaerobic sealant compositions are typically impregnated in the porosity of porous metal parts by wet vacuum impregnation, wet vacuum/pressure impregnation, or dry vacuum/pressure impregnation. These methods are briefly described below with reference to impregnating of porous parts contained in a basket which is introduced into the impregnation chamber, which is the typical method of parts containment if the parts are of suitably small size; in the case of larger parts, the same are typically mounted on or suspended from hoist or other carrier means which is successively translated through the process system including the impregnation chamber.
In the wet vacuum impregnation process, the basket of porous parts is submerged into a vacuum tank of sealant. A short-term, e.g., 10-12 minute, vacuum cycle removes air from the porosity of the parts. The chamber then is returned to ambient pressure, with sealant penetrating into the evacuated porosity. The basket of parts then may be spun briefly in the vacuum tank to allow centrifugal force to eliminate excess sealant waste.
The wet vacuum/pressure impregnation process is similarly conducted, but with the impregnation chamber being pressurized at the end of the vacuum cycle to drive sealant further into small porosity passages.
In the dry vaccum/pressure impregnation method, the basket of porous parts is placed directly in the dry vacuum chamber. Air is evacuated from the porosity in the parts for a selected length of time, e.g., 10 minutes. A transfer valve then is open, allowing sealant to enter the vacuum chamber from a storage reservoir. The chamber is automatically pressurized to force sealant into the parts. After impregnation, while the sealant is being returned to the reservoir, a centrifuge operation spins the basket to remove excess surface sealant.
Among the foregoing methods, wet vacuum impregnation techniques are generally more widely employed than the dry vacuum/pressure impregnation process.
In the above-described impregnation systems, the anaerobic sealant, except during the imposition of vacuum, is continuously aerated to prevent polymerization therof in situ.
Following the initial impregnation step, the impregnated parts are transferred to an agitated water rinse zone, for removal of any remaining surface sealant or sealant trapped in blind holes of the impregnated parts. The agitation of the water rinse zone may be effected by movement of the basket or suspended parts in such zone, and/or mechanical means for effecting circulation of water therein. In the case of small porous parts contained in a basket, it frequently is desirable to operate the water rinse zone in a "tumbling basket" mode to enhance the washing operation and prevent parts with surface sealant deposits which are in contact with one another from sticking together, as a result of polymerization of the contiguous surface sealant deposits.
Following the removal of excess impregnant, impregnated and water-rinsed parts may be transferred in the carrier basket or by conveyor to an activator zone in which the impregnated parts are contacted with a catalyst activator solution, to effect curing of the sealant material at the entrance to the pores in the parts. This creates a hardened plug or cap of sealant material in the outer portion of the pore, trapping the resin for anaerobic self-cure.
Thereafter, the impregnated parts may be transferred to a final rinse zone for removal of the activator solution from the impregnated parts. This final rinse solution may be at elevated temperature, e.g., on the order of about 120.degree. F., to warm the impregnated parts for quick drying, and to accelerate curing of the anaerobic impregnant within the interior porosity of the article, the rate of such cure increasing with increasing temperature.
The final rinse step may also incorporate in the rinse solution suitable rust inhibitor material, for application of a rust inhibiting film to the impregnated article.
As a variation on the above-described impregnation system, it is known to utilize a heat-curing resin in place of the anaerobically-curing resin, whereby the activating and final rinsing steps previously described are eliminated in favor of a hot rinse final step. In the heat-curing resin impregnation system, the parts after impregnation and rinsing of excess surface material, are contacted with hot water at temperatures on the order of 90.degree. C. to cure the impregnant resin.
In all of the above-described impregnation systems, the aqueous washing of the impregnated parts to remove excess surface sealant or sealant trapped in blind holes results in passage of the excess removed resin into the aqueous washing medium. The anaerobic-cure and/or thermal-cure impregnation resins are substantially insoluble in the aqueous washing medium, resulting in the formation of a dispersion or emulsion of the impregnation resin monomer. In order to maximize excess sealant removal action in the aqueous rinse step, clean make-up water is introduced to the rinse tank either continuously or in batch fashion, with corresponding discharge of monomer-containing water from such tank, as waste water effluent.
In conventional impregnation systems of the type described hereinabove, the monomer content of the waste water from the process system may be on the order of from about 0.1 percent up to about 8-10 weight percent (based on the total weight of waste water), or even higher, with concentrations of 0.1-3% being typically encountered.
Heretofore, this monomer-containing waste water effluent of the impregnation process system has either been discharged directly to receiving waters, or else subjected to treatment via conventional biological effluent treatment processes, e.g., activated sludge processing, microbial digestion, etc.
In some instances, the discharge of monomer-containing waste water has resulted in monomer accumulation in the effluent discharge passages and associated valves, tanks, etc., with the result that the agglomerated monomer may experience sufficient lack of oxygen, particularly in the interior of the agglomerated monomeric mass, to cause polymerization to occur. Such build-up of monomer/polymer in the system, if not checked or removed by periodic maintenance, can result in constriction and eventually plugging of effluent water discharge means.
Although the direct discharge or biological treatment of monomer-containing waste water from impregnation systems has been generally satisfactory from an environmental standpoint, there is nonetheless a continuing need to improve the effluent quality of discharge streams from such systems, under the impetus of increasing environmental awareness and legislative and regulatory constraints.
Japanese Patent Application No. 50-47237 filed Apr. 17, 1975 and published Oct. 25, 1976, describes a method for treating eluted waste liquids containing high molecular weight photosensitive resins and low molecular weight reaction monomers, by adding a polymerization initiator to the eluted waste liquid, and subjecting the liquid to heat or light to cause the monomers therein to react (reaction time of 20-40 minutes is disclosed in the specification of this publication, with Example 4 thereof describing photopolymerization by illuminating the waste liquid for one hour with a high-pressure mercury lamp). The reaction product waste liquid then is passed to a concentration tank, from which it is repeatedly circulated through an ultrafiltration apparatus, until concentrated by a factor of 10-15 in the concentration tank. During the ultrafiltration processing of waste liquid, a filtrate is produced which is reused as eluent in the process. When the predetermined concentration in the concentration tank has been achieved, feeding of waste liquid to the concentration tank is discontinued and the ultrafiltration apparatus is shut down. Thereafter, the concentrated waste liquid is mixed with heavy or light oil, and subjected to combustion treatment. The resins disclosed by the patent publication include polyvinyl alcohol, partially hydrolyzed polyvinyl acetate, carbonyl polyvinyl alcohol, hydroxethyl cellulose, gelatine, and water-soluble nylon. The monomers disclosed therein include acrylates or methacrylates with free hydroxyl groups in the alcohol moieties, half esters of acrylic or methacrylic acids of polyethylene glycol, or ones in which the free hydroxyl groups are etherified. As thermal polymerization initiators, there are disclosed persulfates, redox catalysts, ammonium persulfate-sodium thiol, cumene hydroperoxide-cuprous salts, etc., and the disclosed photopolymerization initiators include water-soluble azo compounds and metal ion initiators. The polymerization initiators may be employed at concentrations of 0.05-0.2 parts by weight per 100 parts by weight of the waste liquid, and antifoaming agents such as silicone emulsions may be added to prevent foaming during the developing and eluting process steps.
West Germany Offenlegungsschrift 27 05 159 published Aug. 10, 1978, discloses a process for treating waste waters containing from about 15% to about 80% by weight of emulsified polymerizable liquid substances, based on the total weight of waste water, wherein 0.1-10 weight percent polymerization initiators, based on the total weight of waste water, are added for polymerization of the polymerizable substances in the waste water. The polymerization is carried out at temperatures of between 0.degree. C. and 180.degree. C. and pressures of up to 10 bar, preferably between 50.degree. C. and 95.degree. C. under normal pressure, followed by separation of the polymerization products from the water. This publication discloses the use of polymerization initiators such as peroxides and/or hydroperoxides, and the use of polymerization accelerators such as tertiary amines, organic cobalt, and vanadium salts. The disclosed process is described as applicable to treatment of waste water from impregnation of metal castings with unsaturated polyesters. (Meth)acryl esters are also disclosed as polymerizable emulsifiable monomers to which the disclosed process is applicable.
The waste water treatment process disclosed in the West German patent publication described in the preceding paragraph is disadvantageous, insofar as it effects polymerization of the polymerizable substances therein at elevated temperature, since the resulting waste heat of the waste water must be dissipated so that the final effluent is at near-ambient temperature. This is generally required by environmental regulations, e.g., those promulgated by the U.S. Environmental Protection Agency and various state environmental agencies. If these effluent requirements are not met, the resulting thermal pollution may seriously adversely affect the quality of receiving waters, damage or destroy marine life, etc. Accordingly, when polymerization treatment of monomer-containing waste water is conducted at elevated temperature levels, corresponding refrigeration requirements are imposed on the process system, to achieve the requisite near-ambient final effluent discharge temperatures.
It is therefore an object of the present invention to provide an impregnation system wherein the effluent waste water from the water washing removal of excess resin from the impregnation parts, is treated to be significantly depleted in impregnant monomer content.
It is another object of the invention to provide an impregnation composition for impregnating the porosity of porous articles, which when removed in excess amount from impregnated porous articles by aqueous washing, is readily removable from the aqueous washing/rinse medium.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.